Conventionally, it has been publicly known that EGR gas is introduced to an air intake system of an internal combustion engine. For example, in Patent Document 1, introduction of EGR gas into a compressor of an exhaust turbocharger is disclosed. In this compressor, a circular EGR passage is provided in an outer peripheral area of a housing that houses an impeller. In this circular passage, two recirculation ports are formed for introducing EGR gas into the compressor. The first recirculation port is open in an air passage of an inlet of the impeller. The second recirculation port is open in an air passage on an upstream side of the inlet of the impeller.
EGR gas, which is flown from an exhaust system of an internal combustion engine into a circular passage, flows along an outer periphery of the circular passage, and is flown into a compressor through the first recirculation port or the second recirculation port depending on an amount of intake air flowing in the intake passage. Specifically, when an amount of intake air is small, EGR gas inside the circular passage is flown into the compressor through the first recirculation port. When the amount of intake air is large, EGR gas inside the circular passage is flown into the compressor through the second recirculation port. This is caused by a difference between pressure inside the circular passage and pressure in the inlet of the impeller. In the compressor having such a structure, it is possible to restrain interference between EGR gas and fresh air when the amount of intake air is small, and, when the amount of intake air is large, it is possible to introduce EGR gas directly into the inlet of the impeller.